1. Field of the Invention
This invention relates to the field of process control. More particularly it pertains to automated systems for controlling a heat source during a cure cycle in which a part formed of unpolymerized synthetic resin or B-staged resin impregnated fiber composite material is solidified by polymerizing the resin at elevated temperature.
2. Description of time Prior Art
Fibers such as those of glass, graphite, carbon, boron and organic fibers, such as KEVLAR, are used in combination with resin matrices, such as epoxy, to produce composite materials having particularly desirable properties, especially high stiffness and strength per unit weight.
Composite material is received by a manufacturer from the material supplier in the form of thin sheets called prepreg containing fibers impregnated with resin in a partially cured, intermediate reaction state, the B-stage. In the condition received by a manufacturer from the supplier, the resin reaction, or degree of cure, is about 12 percent completed. Continued reaction of the resin occurs with passage of time and is accelerated with increased temperature; therefore, to retard this reaction, the material is stored until use below room temperature. As the reaction advances, the resin polymerizes, i.e., polymers form and are cross linked until the resin is no longer plastic but solid. Accordingly, the lamina are cut to shape and laid one upon another in sufficient number to achieve a desired thickness while the prepreg is in the B-stage.
Usually the cure process occurs in a pressurized chamber or autoclave into which gas is admitted, heated, and pressurized under closely controlled conditions. The part is surrounded by a mold that defines its contour and shape, and in this environment the resin polymerizes, thereby solidifying the part to the shape of the mold.
The autoclave is pressurized moderately, gas in the autoclave is heated at a predetermined rate, the part temperature rises, and the resin reaction progresses. Pressure is applied and increased to control resin content, part thickness and to minimize porosity, which is the result of voids produced by release of volatile material, such as water vapor, during the curing process.
The chemical reaction during polymerization is exothermal, a release of heat energy. This heat adds to the heat supplied to the part by the ambient atmosphere in the autoclave and can cause overheating. The conventional technique for raising the part temperature in the autoclave is to heat the part slowly and/or to increase the temperature in steps. This method is inefficient, requires a large amount of heat, increases the length of the curing cycle, and requires additional costly tooling to lower processing time.
Various techniques have been used to control by use of a computer the temperature and heat rate of a curing press, furnace or autoclave in which rubber or synthetic resin is molded to shape or cured by heating in a controlled environment.
For example, U.S. Pat. No. 4,344,142 describes a process for determining the time to open a rubber molding press based on the temperature of the press during a heating period. The temperature of the press is determined periodically and read by a computer, which accesses information related to the temperature and time required to cure the compound being molded in the press. The computer determines, from the temperature of the press during the cure period and by calculation of the Arrhenius equation, the optimal time to open the press and terminate the cure. When that time is reached, the press is opened automatically, indicating completion of the cure.
The Arrhenius reaction speed equation is used also in the control system described in U.S. Pat. No. 4,589,072 to determine the chemical reaction amount in a rubber vulcanization reaction. The reaction amount is determined at several places within a reaction system on the basis of temperature signals representing temperatures at those locations. Average, maximum and minimum reaction amount values are determined and used to stop the calculation of reaction amount values when the temperature signal from a location is lower than a predetermined value.
U.S. Pat. Nos. 4,455,268 and 4,515,545 describe systems for controlling the cure process of fiber-reinforced composite materials. The attenuation of an ultrasonic wave introduced into the part being cured is used to determine viscosity of the part, which is compared to reference viscosity data and used to control temperature and pressure of the part in conformance with desired reference values for these variables.
The system of U.S. Pat. No. 4,828,472 controls heating elements and pressurizing devices so that temperature is equalized at various locations on the part being formed of composite material containing resin. The part, enveloped in a bag which is evacuated by a vacuum pump, is located in a pressurized autoclave. The part is heated radiantly by heating elements controlled by a computer. Heat is applied over time according to a predetermined schedule and cooling occurs after sufficient time at a predetermined curing temperature.
In the process described in U.S. Pat. No. 4,810,438 for controlling the cure of fiber-reinforced composite material, the controlling parameter for regulating the variables of the process is the state of the resin as expressed in percent gel, which provides an analytic method to monitor advancement of the resin during the cure. By test, the amount of heat energy required for full gellation is used to indicate the appropriate point where autoclave pressure is to be increased before full gellation.
None of the prior art nor any of these references describes a process for curing parts made of fiber-reinforced composite material in an autoclave with the aid of a digital computer that uses data acquired during processing to calculate repetitively the temperature of the autoclave by accounting for heat generated by the exothermal reaction of the resin during polymerization. The control of the present invention recalculates an optimal autoclave temperature at periodic intervals during the cure cycle on the basis of temperature data acquired at corresponding periodic intervals. The optimal autoclave temperature is determined from repetitively calculated values representing resin heat, maximum offset, lag time and degree of cure of the resin during execution of control algorithms stored in electronic memory accessible to the computer.
While the best mode for carrying out the invention has been described in detail, those familiar with the relevant art will recognize various alternative designs and embodiments for practicing the invention defined by the following claims.